1. Field of the Invention
This invention relates generally to the protection of sensors used on a measuring-while-drilling (MWD) tool, and particularly to a new and improved resistivity antenna shield and wear band assembly that isolates a measuring coil or the like from environmental forces experienced in hostile drilling conditions. The invention also is applicable to a replaceable stabilizer means that substantially centers the tool in a borehole.
2. Description of the Related Art
An MWD measuring tool generally includes a specially made housing or collar having sensors and which is connected in the drill string above the bit. One type of sensor that typically is used is one that enables the electrical resistivity of the rock formations surrounding the borehole to be measured as the hole is deepened by the bit. Mud pulse signals that are representative of such measurements are telemetered uphole where they are detected, processed and displayed and/or recorded as a log of resistivity values versus depth. A resistivity measuring system typically includes one or more transmitting antennas that direct electric current into the formation and two spaced antenna coils that detect returning currents. A comparison of the amplitude or phase shift of the returning current at the receiving coils enables the electrical resistivity of the rock to be determined. Resistivity is a key characteristic in determining whether the rock might contain hydrocarbons.
Directional wells can be drilled with a short radius curved section to establish a new inclination which may bring the borehole to horizontal. As the inclination is rapidly built up, bending of the tool can cause the sensors to engage the borehole wall. When this occurs, friction loads cause rapid wear and other damage so that the sensors can become inoperative. In prior MWD tools, efforts have been made to provide protection for an antenna coil so that it will be more resistant to hostile environmental forces. For example, shields and wear bands having a variety of mechanical fasteners have been employed, all of which are vulnerable to some degree to failure resulting from loosening of such fasteners. Moreover external fasteners that are exposed to high mechanical impact loads against the side of a borehole have been a longstanding weakness in the design of MWD resistivity tools. Thus there is a need to fasten such shields and wear bands in a manner such that the problem of loosening and failure in the borehole is eliminated.
Another component typically used on an MWD tool is a stabilizer which includes a sleeve having a plurality of outwardly directed, longitudinal ribs whose outer faces engage the borehole wall to maintain the sensor collar substantially centered in the borehole. The diameter of such faces can be full-gauge or under-gauge with respect to the gauge diameter of the drill bit, depending upon requirements. It is desirable to mount such a stabilizer on a "slick" collar, that is, a collar without machined upsets for integrally formed threads. If such upsets are not present, the collar would not be destroyed when washed over during a fishing operation. The present invention thus allows more design freedom in placement of sleeves, wear bands and stabilizers.
An object of the present invention is to provide a new and improved antenna coil protection that eliminates the need for external fasteners and thus is more reliable and maintenance free than prior devices.
Another object of the present invention is to provide a new and improved antenna coil protection that can be easily installed in the field in a simple, reliable and maintenance-free manner.
Still another object of the present invention is to provide a new and improved combination of an antenna coil shield with a wear band that provides standoff for the coil.
Yet another object of the present invention is to provide a replaceable sleeve stabilizer that is mounted on a collar by new and improved coupling means that is more reliable and maintenance-free than prior devices for this purpose.